Sour-Gas-Reservoir Exploitation in Croatia

Because fossil fuels are still dominant sources of energy supply, the petroleum industry is called upon not only to provide an effective management of oil and gas reserves in order to meet rising energy demand, but also to do that in a safe and efficient manner, with as small an ecological footprint as practically possible. Consequently, also taking into account the fact that conventional oil and gas reserves are declining, petroleum companies are forced to develop and adopt new technologies to increase oil and gas recovery and to expand their upstream activities to still unexploited areas, which often implies development of deep-buried oil and natural-gas reservoirs characterized by unfavorable reservoir conditions such as high temperature and pressure and even a certain amount of impurities. Croatian experience with natural-gas production from deep-buried reservoirs is based on the development of several gas fields in the northwestern part of Croatia. The development of the largest natural-gas fields in Croatia--Molve, Stari Gradac, and Kalinovac gas fields--began at the beginning of the 1980s. The main characteristic of all the mentioned fields are extremely unfavorable reservoir conditions, with reservoir depth of more than 3000 m, high initial reservoir pressures (more than 450 bar), high temperature (180°C), and a significant share of CO2 (10 to 54%), H2S (800 ppm), and some other nonhydrocarbon compounds such as mercaptans (30 mg/m3) and mercury (1000 to 1500 μg/m3). Several other gas fields with similar reservoir conditions were discovered and developed in the last 25 years in the same region. Today, the petroleum industry in Croatia has almost 30 years of experience in processing sour natural gas with a well-established methodology of auditing processing-plant outlet-gas influences on the environment. These experiences and future plans regarding this subject will be presented in this paper.